On Sat, 05 May 2007 17:55:46 +0100, "Peter Hucker"
wrote:

I need to make a relay switch on 1 second after a 12VDC line comes on, but switch off immediately the 12VDC goes off.
What's the best way to do this? If you need to know why I want this, see the paragraph below:

I have a solar power setup and wish to switch the load between the invertor output and the mains supply.
Currently I have a relay with a 240VAC coil which does the switching based on the output of the invertor.
It doesn't work too good - the invertor output falls a bit as the battery gets low, and the relay starts jumping back and forth.
What I would prefer is to use the 12V signal from the solar regulator (which is on whenever the battery is ok (and is what switches the invertor on).
However I need to wait at least 1 second before moving the load to the invertor, as the invertor takes a while to start up.

Jasen's circuit may well work ok (though I couldn't exactly understand
the ascii circuit - but that's probably my fault).
It may depend on how handy you are at building electronic circuits - I
think there was a 555 timer in it.

There may, however, be a simpler method.
I presume you use a relay to switch on the inverter from the 12v.
You could also use this same voltage to switch two further relays,each
of which must have contacts rated for your mains voltage and current
(these have 12v coils).
The first simply switches the load between mains and the inverter.
However, in the circuit to the inverter, are the contacts of a further
relay, which is time delayed, and is switched by the 12v.

The time delay is formed by using about a 200 ohm coil (lower may work
but will require larger capacitor), and a cap of about 1000uF across
it, with maybe about 20 to 47 ohms in the pos lead to the 12v from the
relay coil. (It might need to be less capacitance)

When the 12v (from the regulator) is off, the inverter will be off and
the n.c.contacts of the first relay will connect the mains to the
load.
When 12v "on", the first relay will switch the load to the wire which
goes to the second relay contact. Because of the time delay, the
output of the inverter will not be connected until relay 2 switches.
This time can be varied by altering the values of C and R.

When 12v switches off again, the load will be immediately switched to
the mains by relay 1. Relay 2 may have a time delay before it "drops
off", but that won't matter, as its is no longer connected to relay1
(nor is there any voltage from the inverter.)

If this isn't clear. I will try a better explanation - but I will be
out of town for two or three days and away from the computer.

Eric Sears.